My interpretation of the above diagram...
blood pressure falls---> the kidneys release renin into the bloodstream.
Renin ( secreted by juxtaglomerular cells) splits angiotensinogen
Angiotensin I--->is split into pieces by angiotensin-converting enzyme (ACE).
Angiotensin II causes the arterioles to constrict, increasing blood pressure.
Angiotensin II triggers the release of aldosterone from the adrenal glands and antidiuretic hormone from the pituitary gland.
Aldosterone causes the kidneys to retain sodium and excrete potassium.
The sodium causes water to be retained, thus increasing blood volume and blood pressure.
